A night vision device amplifies the low levels of light which exist at night to the point where objects become visible to the human eye. The heart of the night vision device is the image intensifier tube, which along with suitable optics make up the night vision device.
The image intensifier tube is comprised of three active components, which are the photocathode, microchannel plate (MCP), and phosphor screen. The photocathode is a photoemissive wafer that is extremely sensitive to low radiation levels of light in the 580-590 nm spectral range. When electromagnetic radiation impinges on the photocathode, the photocathode emits electrons in response.
The MCP is a relatively thin glass plate having input and output planes and an array of microscopic channels through it. An electron impinging on the MCP results in the emission of a number of secondary electrons which, in turn, cause the emission of more secondary electrons. Therefore, each microscopic channel acts as a channel type secondary emission electron multiplier having an electron gain of approximately several hundred. The electron gain is primarily controlled by a potential difference between the input and output planes of the MCP. Consequently, the MCP increases the density of electron emission.
The anode includes an output fiber optic window and a phosphor screen which is formed on a surface of the window. Emitted electrons are accelerated towards the phosphor screen by maintaining the phosphor screen at a higher positive potential than the MCP. The phosphor screen converts the electron emission into an image which is visible to an operator. The tube body housing encompasses all three of the “active” tube elements so a vacuum envelope can be maintained.
As with most electronic devices, it is desired for the image intensifier to be as small as possible. The standard sizes presently have diameters of 18 mm and 25 mm. The miniaturization would be in the form decreasing the length, and weight of the image intensifier tube. The active tube components that can be shortened in length and diameter to any measurable degree would be the photocathode and the phosphor screen. The MCP can be changed in diameter but the length change would be at an absolute minimum.
A problem associated with the present design of image intensifier tubes is that the known methods for holding or locking the MCP in the tube housing add length and diameter to the tube. That is, the fill factor (active components in proportion to support components) is very poor. The two main prior art methods of locking the MCP in the tube housing are described below in connection with FIGS. 1 and 2.
It is thus an object of the present invention to provide an image intensifier tube having a device for locking the MCP which does not add length to the tube and reduces the diameter.
In accordance with an aspect of the invention, this object is accomplished by providing an image intensifier tube comprising a tube housing which holds a photocathode and a screen, wherein a collar is provided which retains an MCP in a recess in an interior surface of the collar.